This invention relates to improved and simplified boom support structure for large dragline excavators.
In the past, a gantry has been mounted upon a deck, or main frame, of an excavating machine, with the gantry rising upwardly from the deck to support a boom by means of tension members running generally from the top of the gantry to the point of the boom. The gantry consists of front and back legs connected to the deck, with the front legs rising substantially vertically and the point of intersection of the front and back legs defining the top to which the boom-supporting tension members are connected. The boom has its foot secured at widely spaced points on the deck, usually directly over the circular rail beneath the frame which rests upon a circle of rollers to allow the main frame to swing, or rotate, about a vertical axis. The gantry front legs are widespread, and are mounted to the deck closely adjacent the boom foot pins.
With the gantry and boom structure just described, excessive loading occurs at the locations of the boom foot pins and gantry front leg mountings, which causes increased wear and pitting of the rollers and rail circle at the points of excessive loading. The front end of the main frame has insufficient stiffness for adequately resisting deflection and uniformly distributing the boom weight and hoist load imposed thereon over a segment of roller circle disposed beneath the front end of the main frame. At the front center area of the rail circle, there is little or no loading due to main frame deflection. This uneven distribution of weight and load results in uneven wear and inefficient operation.
A gantry such as described must be erected piece by piece in the field. This means that much fitting and welding has to be done at high elevations, and a large derrick is needed for long periods of time in placing small items in the upper portions of the gantry. This method is expensive and time consuming.
In large draglines, it is customary to employ some form of fairlead assembly to direct the drag ropes from the bucket to the drum so as to prevent the ropes from unnecessary wear by scraping against stationary surfaces, or by excessive flight angles with respect to the sheaves or drum. These have taken different forms, and with one form, known as the over and under fairlead, the fairlead and hoist deflecting sheaves are mounted in a tower, with the top of the tower being pinned to the front legs of the gantry and the tower bottom being connected to the excavator deck, or main frame. Unfortunately, the gantry is complicated by extensive truss structure just below the house roof to support rails for traveling cranes inside the house, which cranes are used for lifting and moving equipment. A further complication arises from the fact that the gantry front legs, at least in part, are within the house, requiring flashing at the points of exit to avoid water leakage. These factors contribute to the difficulty in suitably locating the fairlead tower top relative to the gantry. In addition, the gantry and fairlead tower require multiple points of attachment to the deck on multiple axes.
The prior structure results in boom weight and hoist loads being taken by the gantry, while drag loads are taken by the fairlead tower. The use of the two structures for the separate loads causes complication, expense, and loss of peak operational performance.